1. Field of the Invention
This invention relates to the practice of operating main and distribution networks of pressure and gravity flow steel pipelines. More particularly, it concerns a method of protecting the inside surface of a pipeline from deposits and applying a protective coating thereto.
2. Description of the Invention
The prior art knows a range of methods for cleaning the inside of pipelines or removing depsits therefrom. However, while solving the problem of pipe cleaning, these fail to provide reliable protection of the pipeline inside walls from corrosion along with cleaning.
For example, there is known a method of cleaning pipeline interior accompanied by protecting the inside walls of the pipeline from subsequent contamination in which a protective layer is formed in the course of cleaning by removing excessive deposits and compacting the remaining deposit layer with all the pipeline improvement operations executed mechanically (cf., USSR Inventor's Certificate No. 1.018,729, IPC B 08 17/00, published 1982). However, this method fails to prevent pitting which affects the pipeline surface, because pits remain in the thus compacted layer.
Widely popular is a hydromechanical method of removing deposits from the interior of pipelines, which resides in that a self-contained pipe-cleaning unit is moved along the pipeline under a liquid overpressure, whereby part of the liquid passed through this unit acts to entrain the deposits scrubbed away from the pipeline walls (cf., USSR Inventor's Certificate No. 856,599, IPC B 08 B 9/04; and USSR Inventor's Certificate No. 716,647, IPC B 08 B 9/04, published 1976).
Although this method ensured inside pipe wall cleaning to the pipe metal, the extent to which pipe deposits are removed is not sufficient for applying a corrosion-resistant coating, because pits and flaws of the pipe inside surface retain some corrosive deposits conductive to subsequent pitting. Also, another disadvantage of the method includes the susceptibility of the freshly cleaned and unprotected metal surface of the pipe to vigorous corrosion, while pipe thickness reduces due to the loss of metal.
Provision of a protective layer on the pipe surface cleaned from deposits is possible in a number of ways. There is known, for example, a method of treating such cleaned pipe surface for a period of six days by a solution of corrosion inhibitor composed of a high concentration sodium polyphosphate (75 mg/l in terms of P.sub.2 O.sub.5), after which the thus formed coating is continuously replenished with a weaker solution of sodium polyphosphate (5 mg/l P.sub.2 O.sub.5) (cf., Klyachko V.A. and Apeltsin I.E. "Ochistka prirodnykh yod"--Natural Water Purification, in Russian, the "Stroiizdat" Publishers, 1971, pp. 507 to 512).
However, such inhibitor fails to protect drinking water pipelines from corrosion, since the amount of sodium polyphosphate continuously added to the flow of liquid carried by the p peline exceeds that prescribed for drinking water. Conversely, without continuously adding the inhibitor the coating exhibits signs of rapid wear. Otherwise stated, it is necessary that a definite ratio between the contents of calcium and sodium polyphosphate be maintained throughout pipeline operation, or the sodium polyphosphate promote corrosion.
In view of the aforedescribed, none of the known methods of chemically protecting a pipeline from corrosion provides reliable and long-lasting coatings.